Marshrock formed by iron sulphide and siderite cementation in saltmarsh sediments

Abstract

The formation of authigenic sulphides and other iron minerals has proved interesting both in studies of palaeomagnetism¹ and palaeosalinity reconstructions of Pleistocene and older sediments^2,3. Iron sulphide and siderite-cemented nodules, concretions and layers are fairly common in the geological record^4–6, but there is little evidence of their formation in modern environments. Accounts of authigenic iron minerals forming today in marine and brackish sediments generally refer only to ‘micro-concretions’ 10–500 µm in size, many of which have a ‘framboidal’ texture^3,7,8. I describe here ‘macro-concretions’ and cemented layers which are now forming within intertidal marsh sediments at Warham, Norfolk, UK. Parts of these marshes are only 30 yr old, indicating how quickly early diagenetic cementation can occur. Siderite (FeCO₃), greigite (Fe₃S₄) and possibly mackinawite (FeS_1-x) are the main cementing agents. The term ‘marshrock’ is proposed for the cemented sediments by analogy with calcareous beachrock^9,10. The formation of siderite–monosulphide concretions in the Warham sediments probably reflects a high ‘reactive iron’ content and limited sulphate arising from rapid sedimentation and burial of organic matter during the early stages of marsh growth.